In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional unit, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is selected in the transmission mechanism.
Each of the frictional units thus operative is actuated to couple or apply by a control fluid pressure, usually called line pressure, developed by a pressure regulator valve incorporated in the hydraulic control system. If the line pressure is supplied at an uncontrolled rate to any one or two of these fluid operated frictional units during shifting from the neutral gear position to the automatic forward drive range position or the reverse drive gear position, mechanical shocks tend to be created in the transmission mechanism and will give the occupants of the vehicle an unpleasant sensation.
The brake band to produce the second forward drive gear ratio is automatically operated to apply or release by means of a fluid operated band servo unit depending upon the vehicle speed under automatic forward drive range condition. If the line pressure is supplied to the band servo unit at an uncontrolled rate during shifting from the first gear ratio to the second gear ratio in the automatic forward drive range, mechanical shocks are also produced in the transmission mechanism.
The present invention contemplates elimination or reduction of these mechanical shocks usually concomitant with the selection of the automatic forward drive range or reverse drive gear position or upshifting between the automatic first and second forward drive gear positions by the use of a single pressure accumulator unit incorporated into a shock control arrangement.
In order to alleviate the shocks to be produced during upshifting between the first and second gear ratios in the automatic forward drive range, it is important that a pressure accumulator unit provided to reduce such shocks be initiated into motion to take up the band actuating fluid pressure in such a manner that the smaller the load on the engine, the lower will the band actuating fluid pressure be at which the pressure accumulator unit is initiated into motion to take up the fluid pressure. If, on the contrary, the pressure accumulator is initiated into motion to take up the band actuating fluid pressure in response to a band actuating fluid pressure of a level predetermined without respect to the load on the engine, the shocks to be produced during upshifting between the automatic first and second forward gear ratios could not be reduced or eliminated by the use of a pressure accumulator because the band actuating fluid pressure may have augmented beyond an allowable limit by the point of time when the accumulator unit is put into operation under low to medium load operating conditions of the engine. If the accumulator unit commences taking up the band actuating fluid pressure at a stage when the increase in the fluid pressure is premature, then the brake band would tend to slip on the brake drum onto which the band is to be applied. This will result in accelerated wear of the band and the brake drum and will impair the service life thereof.
The present invention has been completed on the basis of the conception that the line pressure to act on the frictional units of a transmission mechanism is developed in relation to a throttle pressure continuously variable with the opening degree of an engine and accordingly with the load on the engine and that the level of the band actuating fluid pressure at which a pressure accumulator unit is to be initiated into motion can be made variable with load on the engine if the line pressure is utilized in the pressure accumulator unit in such a manner as to oppose the band actuating fluid pressure.